Yes there is a limit: 1,000,000. But even if you do hit that number i won't delete some to make room. It sounds more like you have a corrupted database.
Back Up and try rebuild the library: hold down the command and option (or alt) keys while launching iPhoto. Use the resulting dialogue to rebuild. Choose to Repair Database. If that doesn't help, then try again, this time using Rebuild Database.
If that fails:
Download iPhoto Library Manager and use its rebuild function. (In Library Manager it's the FIle -> Rebuild command)
This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.
Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.
Terence, thank for your support. I tried both options, without success. The difference is, that in the old file the miniatures of the missing photos are still available, but not the originals. In the rebuilt file, both are missing.
Due to system problems, I am unable to add the other screen shots.
When I double click the miniature of a missing photo, a black screen with a triangle an a exclamation-mark appears. Only photos originated 2010 to 2011 disappeared, the older ones are still available. It could be that an account change in 2010 caused the problem. I moved the entire iPhoto library from one account to another.
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Yes, I have a kind of a backup because I distributed most of the photos within my family. My own backup is affected, too.
It is strange that the miniatures are still available. My main issue to prevent such problems in future. What I learned is to keep a copy of the photo in a separate folder outside iPhoto.